Lay two 9-inch pieces of heavy-duty foil on top of each other and place on a rimless baking sheet. Arrange lemon slices in two layers in the center of the foil. Spread herb sprigs over the lemons. With the side of a chef’s knife, mash garlic with salt to form a paste. Transfer to a small dish and stir in mustard and the remaining 2 tablespoons chopped herbs. Spread the mixture over both sides of the salmon. Place the salmon on the herb sprigs.

Slide the foil and salmon off the baking sheet onto the grill without disturbing the salmon-lemon stack. Cover the grill; cook until the salmon is opaque in the center, 18 to 24 minutes. Wearing oven mitts, carefully transfer foil and salmon back onto the baking sheet. Cut the salmon into 4 portions and serve with lemon wedges (discard herb sprigs and lemon slices).